Asteroid belts at just the right place are friendly to life

November 6, 2012

This illustration shows three possible scenarios for the evolution of asteroid belts. Top: a Jupiter-size planet migrates through the asteroid belt, scattering material and inhibiting the formation of life on planets. Middle: our solar-system model: a Jupiter-size planet that moves slightly inward but is just outside the asteroid belt. Bottom: a large planet does not migrate at all, creating a massive asteroid belt. Material from the hefty asteroid belt would bombard planets, possibly preventing life from evolving. (Credit: NASA/ESA/STScI)

Solar systems with life-bearing planets may be rare if they are dependent on the presence of asteroid belts of just the right mass, according to astronomers.

They suggest that the size and location of an asteroid belt, which is shaped by the evolution of the sun’s planet-forming disk and by the gravitational influence of a nearby giant Jupiter-like planet, may determine whether complex life will evolve on an Earth-like planet.

That’s because asteroids may have delivered water and organic compounds to the early Earth. According to the theory of punctuated equilibrium, occasional asteroid impacts might also accelerate the rate of biological evolution by disrupting a planet’s environment to the point where species must try new adaptation strategies.

The astronomers based their conclusion on an analysis of theoretical models and archival observations, including infrared data from NASA’s Spitzer Space Telescope.

Rebecca Martin, a NASA Sagan Fellow from the University of Colorado in Boulder, and astronomer Mario Livio of the Space Telescope Science Institute in Baltimore, Md. suggest that the location of an asteroid belt relative to a Jupiter-like planet is not an accident.

“Our study shows that only a tiny fraction of planetary systems observed to date seem to have giant planets in the right location to produce an asteroid belt of the appropriate size, offering the potential for life on a nearby rocky planet,” said Martin, the study’s lead author. “Our study suggests that our solar system may be rather special.”

The asteroid belt in our solar system, located between Mars and Jupiter, is a region of millions of space rocks that sits near the “snow line,” which marks the border of a cold region where volatile material such as water ice is far enough from the sun to remain intact. When Jupiter formed just beyond the snow line, its powerful gravity prevented nearby material inside its orbit from coalescing and building planets.

Instead, Jupiter’s influence caused the material to collide and break apart. These fragmented rocks settled into an asteroid belt around the sun.

“To have such ideal conditions you need a giant planet like Jupiter that is just outside the asteroid belt [and] that migrated a little bit, but not through the belt,” Livio explained. “If a large planet like Jupiter migrates through the belt, it would scatter the material. If, on the other hand, a large planet did not migrate at all, that, too, is not good because the asteroid belt would be too massive. There would be so much bombardment from asteroids that life may never evolve.”

Using our solar system as a model, Martin and Livio proposed that asteroid belts in other solar systems would always be located approximately at the snow line. To test their proposal, Martin and Livio created models of planet-forming disks around young stars and calculated the location of the snow line in those disks based on the mass of the central star.

They then looked at all the existing space-based infrared observations from the Spitzer Space Telescope of 90 stars having warm dust, which could indicate the presence of an asteroid belt-like structure. The temperature of the warm dust was consistent with that of the snow line. “The warm dust falls right onto our calculated snow lines, so the observations are consistent with our predictions,” Martin said.

The duo then studied observations of the 520 giant planets found outside our solar system. Only 19 of them reside outside the snow line. This suggests that most of the giant planets that may have formed outside the snowline have migrated too far inward to preserve the kind of slightly dispersed asteroid belt needed to foster enhanced evolution of life on an Earth-like planet near the belt. Apparently, less than four percent of the observed systems may actually harbor such a compact asteroid belt.

“Based on our scenario, we should concentrate our efforts to look for complex life in systems that have a giant planet outside of the snow line,” Livio said.

The findings will appear today in the Monthly Notices of the Royal Astronomical Society: Letters.

The Sagan Fellowship Program is administered by the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena, Calif., whose purpose is to advance the scientific and technical goals of NASA’s Exoplanet Exploration Program. The Exoplanet Exploration Program is managed for NASA by NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Caltech manages JPL for NASA.

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The team proposed that asteroid belts in other solar systems would always be located about at the snow line. In order to test their hypothesis, they created models of planet-forming disks around young stars, and calculated the location of the snow line in those disks based on the mass of the central star.

Unfortunately the science of this sounds credible. It’s a shame. It will greatly reduce the number of habitable planets. On the bright side, we will be able to narrow down the possible planets, for closer study.

This is an important paper. I am going to save it to Word and store it in Documents, on a DVD, and in a handful of flash drives.

By the way, if this interests you, go to Space.com and look at the article about Neil deGrasse Tyson appearing in a Superman comic to tell that Krypton was located in the constellation Corvus. He even gives the catalog number of the red dwarf he believes was the doomed world’s home star. It was a fun article, check it out.